Methods of forming a phase change memory with vertical cross-point structure

ABSTRACT

A non-volatile memory uses phase change memory (PCM) cells in a three dimensional vertical cross-point structure, in which multiple layers of word lines run in a horizontal direction and bit lines run in a vertical direction. The memory cells are located in a recessed region of the word lines and are separated from the bit line by an ovonic threshold switch. A surfactant lining of the word line recess in which the phase change memory material is placed improves stability of the resistance state of the memory cells, allowing for improved multi-state operation.

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 62/572,831, filed Oct. 16, 2017, which isincorporated by reference herein in its entirety for all purposes.

BACKGROUND

Semiconductor memory is widely used in various electronic devices suchas mobile computing devices, mobile phones, solid-state drives, digitalcameras, personal digital assistants, medical electronics, servers, andnon-mobile computing devices. Semiconductor memory may comprisenon-volatile memory or volatile memory. A non-volatile memory deviceallows information to be stored or retained even when the non-volatilememory device is not connected to a source of power (e.g., a battery).Examples of non-volatile memory include flash memory (e.g., NAND-typeand NOR-type flash memory), Electrically Erasable Programmable Read-OnlyMemory (EEPROM), ferroelectric memory (e.g., FeRAM), magnetoresistivememory (e.g., MRAM), and phase change memory (e.g., PCM). In recentyears, non-volatile memory devices have been scaled in order to reducethe cost per bit. However, as process geometries shrink, many design andprocess challenges are presented. These challenges include increasedvariability in memory cell characteristics over process, voltage, andtemperature variations and increased leakage currents through unselectedmemory cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1F depict various embodiments of a memory system.

FIGS. 2-3 depict various embodiments of a portion of a three-dimensionalmemory array.

FIG. 4 depicts various embodiments of a portion of a three-dimensionalmemory array with a vertical cross-point structure.

FIG. 5 depicts one embodiment of a read/write circuit.

FIGS. 6A-C illustrate a phase change memory (PCM) memory cell withmultiple resistance levels.

FIG. 7A is cross-sectional side view that shows one embodiment of avertical cross-point memory structure with recessed PCM memory cells.

FIG. 7B is top view of one embodiment of a vertical cross-point memorystructure with recessed PCM memory cells.

FIGS. 7C and 7D illustrate current flow from a word line into a memorycell for the “crested” structure of FIG. 7B and a non-“crested” wordline structure, respectively.

FIG. 8 is an equivalent circuit diagram representation of the memorystructure of FIG. 7A.

FIG. 9A shows one embodiment for write circuitry.

FIG. 9B illustrates examples of current pulses for multi-stateprogramming of a PCM memory cell.

FIG. 9C is a flowchart describing an embodiment for writing an array ofPCM based memory cells.

FIG. 9D illustrates an embodiment for a current pulse to set the memorycells' PCM to the crystalline phase.

FIG. 9E is a flowchart describing an embodiment for erasing an array ofphase change memory material based memory cells in which the memorycells are set to a crystalline state.

FIG. 10A illustrates the current-voltage characteristics of a PCM basedmemory cell.

FIG. 10B shows one embodiment for read circuitry such as can beincorporated in the read circuit of FIG. 5.

FIG. 10C is a flowchart describing an embodiment for reading an array ofmulti-state PCM material based memory cells.

FIGS. 11A-11L illustrate various processing stages for the memorystructure of FIGS. 7A and 7B.

FIG. 12 depicts a flowchart describing embodiments of processes forforming portions of a memory array as in FIGS. 7A and 7B.

DETAILED DESCRIPTION

A non-volatile memory uses phase change memory (PCM) cells in a threedimensional vertical cross-point structure, in which multiple layers ofword lines run in a horizontal direction and bit lines run in a verticaldirection. The memory cells are located in a recessed pocket region, or“pore”, of the word lines and are separated from the bit line by anovonic threshold switch or other selector device. A surfactant lining ofthe word line pocket in which the phase change memory material isconfined improves stability of the resistance state of the memory cells,allowing for improved multi-state operation.

A phase change memory material, such as a chalcogenide material likeGe₂Sb₂Te₅ (GST), may change phases from an amorphous state (e.g.,corresponding with a high resistance “reset” state) to a crystallinestate (e.g., corresponding with a low resistance “set” state) or from aless-ordered crystalline state to a more-ordered crystalline state. ThePCM material may initially be in a crystalline phase, low-resistancestate after fabrication. Thereafter, the PCM material or portion of thePCM material may be reset into an amorphous phase, high-resistance stateby melting a portion of the PCM material and then quickly quenching theportion of the PCM material by applying a current pulse through the PCMmaterial for a first period of time. The amorphous portion of the PCMmaterial in series with any crystalline region of the PCM material maydetermine the effective resistance of the PCM material. One side of thePCM material may connect to a first electrode, such as a word line, anda portion on the other side may connect to a second electrode, such as abit line. The current passing through the PCM material may cause asubstantial amount of heat to be generated due to current. The amount ofheat generated may be proportional to the amount of current passingthrough the memory cell. The PCM material may be set into a crystallinephase, low-resistance state by applying a current pulse to the PCMmaterial to anneal the amorphous portion of the PCM material at atemperature between the crystallization temperature for the PCM materialand the melting temperature for the PCM material for a second period oftime to crystallize the amorphous portion of the PCM material. Thesecond period for setting the PCM material may be greater than the firstperiod of time for resetting the PCM material.

By varying the amount of the PCM material that is in the amorphous staterelative to the crystalline state, multi-state data can be stored in aPCM material based memory cell. For example, starting in the crystallinestate pulses of different current levels can program the memory cellinto different data states by placing different amounts of the memorycell's PCM material in the amorphous state. As the resistivity of theamorphous state tends to be relatively unstable with respect totemperature or other conditions, the number of states that can bepractically stored in a PCM based memory cell can be limited. To improvethis situation, the PCM material can be formed within a confined pocketor pore structure that is lined with a surfactant. If the surfactant hasa relatively stable resistivity that is higher than the crystallinestate, but lower than amorphous state, the surfactant can provide aparallel current path around the amorphous region, allowing for morestable multi-state memory cell than using the PCM material alone.

In some embodiments, a memory array may comprise a cross-point memoryarray structure. A cross-point memory array may refer to a memory arrayin which two-terminal memory cells are placed at the intersections of afirst set of control lines (e.g., word lines) arranged in a firstdirection and a second set of control lines (e.g., bit lines) arrangedin a second direction perpendicular to the first direction. In somecases, each memory cell in a cross-point memory array may be placed inseries with a selection device acting as a steering element or anisolation element, such as a diode or ovonic threshold switch, in orderto reduce leakage currents. The structure can be formed as a series ofalternating dielectric and word line layers, where the word lines layerscan have a series of prongs, forming a crested comb-like structure. Fromthe holes or openings etched down into the dielectric layers for the bitlines, the prongs can be etched back to form the pocket or pore regionthat can be lined with the surfactant and then filed with the phasechange memory material for each of the memory cells. The vertical bitline structure, along with the ovonic or other selection switches, canthen be formed to with the holes.

In some embodiments, a three-dimensional memory array may include one ormore vertical columns of memory cells located above and orthogonal to asubstrate. In one example, a non-volatile storage system may include amemory array with vertical bit lines or bit lines that are arrangedorthogonal to a semiconductor substrate. The substrate may comprise asilicon substrate. The memory array may include rewriteable non-volatilememory cells, wherein each memory cell includes PCM material having areversible resistance-switching element that can be programmed tomultiple resistance levels.

In some embodiments, a non-volatile storage system may include anon-volatile memory that is monolithically formed in one or morephysical levels of arrays of memory cells having an active area disposedabove a silicon substrate. The non-volatile storage system may alsoinclude circuitry associated with the operation of the memory cells(e.g., decoders, state machines, page registers, or control circuitryfor controlling the reading and/or programming of the memory cells). Thecircuitry associated with the operation of the memory cells may belocated above the substrate or located within the substrate. In someembodiments, the multi-state memory pPCM memory cells can be read byusing a current mirror to compare the current running through the cellsin response to different voltage levels with a reference current. Insome embodiments, multi-state data can be written to the phase changememory material based memory cell by using a current mirror to applycurrent pulses of different amplitudes to the memory cell.

FIG. 1A depicts one embodiment of a host 106 and a memory system 101 inwhich the phase change memory can be implemented. The memory system 101may comprise a non-volatile storage system interfacing with the host(e.g., a mobile computing device or a server). In some cases, the memorysystem 101 may be embedded within the host 106. As examples, the memorysystem 101 may comprise a memory card, a solid-state drive (SSD) such ahigh-density MLC (multi-level, or multi-state, cell) SSD (e.g.,2-bits/cell or 3-bits/cell) or a high performance SLC (single levelcell) SSD, or a hybrid HDD/SSD drive. As depicted, the memory system 101includes a memory chip controller 105 and a memory chip 102. The memorychip 102 may include volatile memory and/or non-volatile memory.Although a single memory chip is depicted, the memory system 101 mayinclude more than one memory chip (e.g., four or eight memory chips).The memory chip controller 105 may receive data and commands from host106 and provide memory chip data to host 106. The memory chip controller105 may include one or more state machines, page registers, SRAM, andcontrol circuitry for controlling the operation of memory chip 102. Theone or more state machines, page registers, SRAM, and control circuitryfor controlling the operation of the memory chip may be referred to asmanaging or control circuits. The managing or control circuits mayfacilitate one or more memory array operations including forming,erasing, programming, or reading operations.

In some embodiments, the managing or control circuits (or a portion ofthe managing or control circuits) for facilitating one or more memoryarray operations may be integrated within the memory chip 102. Thememory chip controller 105 and memory chip 102 may be arranged on asingle integrated circuit or arranged on a single die. In otherembodiments, the memory chip controller 105 and memory chip 102 may bearranged on different integrated circuits. In some cases, the memorychip controller 105 and memory chip 102 may be integrated on a systemboard, logic board, or a PCB.

The memory chip 102 includes memory core control circuits 104 and amemory core 103. Memory core control circuits 104 may include logic forcontrolling the selection of memory blocks (or arrays) within memorycore 103, controlling the generation of voltage references for biasing aparticular memory array into a read or write state, and generating rowand column addresses. The memory core 103 may include one or moretwo-dimensional arrays of memory cells or one or more three-dimensionalarrays of memory cells. In one embodiment, the memory core controlcircuits 104 and memory core 103 may be arranged on a single integratedcircuit. In other embodiments, the memory core control circuits 104 (ora portion of the memory core control circuits) and memory core 103 maybe arranged on different integrated circuits.

Referring to FIG. 1A, a memory operation may be initiated when host 106sends instructions to memory chip controller 105 indicating that itwould like to read data from memory system 101 or write data to memorysystem 101. In the event of a write (or programming) operation, host 106may send to memory chip controller 105 both a write command and the datato be written. The data to be written may be buffered by memory chipcontroller 105 and error correcting code (ECC) data may be generatedcorresponding with the data to be written. The ECC data, which allowsdata errors that occur during transmission or storage to be detectedand/or corrected, may be written to memory core 103 or stored innon-volatile memory within memory chip controller 105. In oneembodiment, the ECC data is generated and data errors are corrected bycircuitry within memory chip controller 105.

Referring to FIG. 1A, the operation of memory chip 102 may be controlledby memory chip controller 105. In one example, before issuing a writeoperation to memory chip 102, memory chip controller 105 may check astatus register to make sure that memory chip 102 is able to accept thedata to be written. In another example, before issuing a read operationto memory chip 102, memory chip controller 105 may pre-read overheadinformation associated with the data to be read. The overheadinformation may include ECC data associated with the data to be read ora redirection pointer to a new memory location within memory chip 102 inwhich to read the data requested. Once a read or write operation isinitiated by memory chip controller 105, memory core control circuits104 may generate the appropriate bias voltages for word lines and bitlines within memory core 103, as well as generate the appropriate memoryblock, row, and column addresses.

In some embodiments, one or more managing or control circuits may beused for controlling the operation of a memory array within the memorycore 103. The one or more managing or control circuits may providecontrol signals to a memory array in order to perform a read operationand/or a write operation on the memory array. In one example, the one ormore managing or control circuits may include any one of or acombination of control circuitry, state machines, decoders, senseamplifiers, read/write circuits, and/or controllers. The one or moremanaging circuits may perform or facilitate one or more memory arrayoperations including erasing, programming, or reading operations. In oneexample, one or more managing circuits may comprise an on-chip memorycontroller for determining row and column address, word line and bitline addresses, memory array enable signals, and data latching signals.

FIG. 1B depicts one embodiment of memory core control circuits 104. Asdepicted, the memory core control circuits 104 include address decoders170, voltage generators for selected control lines 172, and voltagegenerators for unselected control lines 174. Control lines may includeword lines, bit lines, or a combination of word lines and bit lines.Selected control lines may include selected word lines or selected bitlines that are used to place memory cells into a selected state.Unselected control lines may include unselected word lines or unselectedbit lines that are used to place memory cells into an unselected state.The voltage generators (or voltage regulators) for selected controllines 172 may comprise one or more voltage generators for generatingselected control line voltages. The voltage generators for unselectedcontrol lines 174 may comprise one or more voltage generators forgenerating unselected control line voltages. Address decoders 170 maygenerate memory block addresses, as well as row addresses and columnaddresses for a particular memory block.

FIGS. 1C-1F depict one embodiment of a memory core organization thatincludes a memory core having multiple memory bays, and each memory bayhaving multiple memory blocks. Although a memory core organization isdisclosed where memory bays comprise memory blocks, and memory blockscomprise a group of memory cells, other organizations or groupings canalso be used with the technology described herein.

FIG. 1C depicts one embodiment of memory core 103 in FIG. 1A. Asdepicted, memory core 103 includes memory bay 330 and memory bay 331. Insome embodiments, the number of memory bays per memory core can bedifferent for different implementations. For example, a memory core mayinclude only a single memory bay or a plurality of memory bays (e.g., 16memory bays or 256 memory bays).

FIG. 1D depicts one embodiment of memory bay 330 in FIG. 1C. Asdepicted, memory bay 330 includes memory blocks 310-312 and read/writecircuits 306. In some embodiments, the number of memory blocks permemory bay may be different for different implementations. For example,a memory bay may include one or more memory blocks (e.g., 32 memoryblocks per memory bay). Read/write circuits 306 include circuitry forreading and writing memory cells within memory blocks 310-312. Asdepicted, the read/write circuits 306 may be shared across multiplememory blocks within a memory bay. This allows chip area to be reducedsince a single group of read/write circuits 306 may be used to supportmultiple memory blocks. However, in some embodiments, only a singlememory block may be electrically coupled to read/write circuits 306 at aparticular time to avoid signal conflicts.

In some embodiments, read/write circuits 306 may be used to write one ormore pages of data into the memory blocks 310-312 (or into a subset ofthe memory blocks). The memory cells within the memory blocks 310-312may permit direct over-writing of pages (i.e., data representing a pageor a portion of a page may be written into the memory blocks 310-312without requiring an erase or reset operation to be performed on thememory cells prior to writing the data). In one example, the memorysystem 101 in FIG. 1A may receive a write command including a targetaddress and a set of data to be written to the target address. Thememory system 101 may perform a read-before-write (RBW) operation toread the data currently stored at the target address before performing awrite operation to write the set of data to the target address. Thememory system 101 may then determine whether a particular memory cellmay stay at its current state (i.e., the memory cell is already at thecorrect state), needs to be set to a “0” state, or needs to be reset toa “1” state. The memory system 101 may then write a first subset of thememory cells to the “0” state and then write a second subset of thememory cells to the “1” state. The memory cells that are already at thecorrect state may be skipped over, thereby improving programming speedand reducing the cumulative voltage stress applied to unselected memorycells. A particular memory cell may be set to the “1” state by applyinga first voltage difference across the particular memory cell of a firstpolarity (e.g., +1.5V). The particular memory cell may be reset to the“0” state by applying a second voltage difference across the particularmemory cell of a second polarity that is opposite to that of the firstpolarity (e.g., −1.5V). The reading and writing of phase change memoryis described further below.

In some cases, read/write circuits 306 may be used to program aparticular memory cell to be in one of three or more data/resistancestates (i.e., the particular memory cell may comprise a multi-statememory cell). In one example, the read/write circuits 306 may apply afirst voltage difference (e.g., 2V) across the particular memory cell toprogram the particular memory cell into a first state of the three ormore data/resistance states or a second voltage difference (e.g., 1V)across the particular memory cell that is less than the first voltagedifference to program the particular memory cell into a second state ofthe three or more data/resistance states. Applying a smaller voltagedifference across the particular memory cell may cause the particularmemory cell to be partially programmed or programmed at a slower ratethan when applying a larger voltage difference. In another example, theread/write circuits 306 may apply a first voltage difference across theparticular memory cell for a first time period (e.g., 150 ns) to programthe particular memory cell into a first state of the three or moredata/resistance states or apply the first voltage difference across theparticular memory cell for a second time period less than the first timeperiod (e.g., 50 ns). One or more programming pulses followed by amemory cell verification phase may be used to program the particularmemory cell to be in the correct state. In other embodiments, pulses ofdifferent current levels can be used, were the read/write circuits 306used pulses of different current levels to write the memory cells to thedifferent target data states. For phase change memory (PCM) memoryembodiments, a memory cell can be moved from the lower resistivitycrystalline state to states of increasing levels of amorphous state byapplying current pulses of higher current levels or durations to raiseportions of the PCM material above the melt temperature. The PCM memorycells can be moved to a more crystalline state by the read/writecircuits 306 by applying current pulses sufficient to raise the PCMmaterial above its crystallization temperature, but below its melttemperature.

FIG. 1E depicts one embodiment of memory block 310 in FIG. 1D. Asdepicted, memory block 310 includes a memory array 301, row decoder 304,and column decoder 302. Memory array 301 may comprise a contiguous groupof memory cells having contiguous word lines and bit lines. Memory array301 may comprise one or more layers of memory cells. Memory array 310may comprise a two-dimensional memory array or a three-dimensionalmemory array. The row decoder 304 decodes a row address and selects aparticular word line in memory array 301 when appropriate (e.g., whenreading or writing memory cells in memory array 301). The column decoder302 decodes a column address and selects a particular group of bit linesin memory array 301 to be electrically coupled to read/write circuits,such as read/write circuits 306 in FIG. 1D. In one embodiment, thenumber of word lines is 4K per memory layer, the number of bit lines is1K per memory layer, and the number of memory layers is 4, providing amemory array 301 containing 16M memory cells.

FIG. 1F depicts one embodiment of a memory bay 332. Memory bay 332 isone example of an alternative implementation for memory bay 330 in FIG.1D. In some embodiments, row decoders, column decoders, and read/writecircuits may be split or shared between memory arrays. As depicted, rowdecoder 349 is shared between memory arrays 352 and 354 because rowdecoder 349 controls word lines in both memory arrays 352 and 354 (i.e.,the word lines driven by row decoder 349 are shared). Row decoders 348and 349 may be split such that even word lines in memory array 352 aredriven by row decoder 348 and odd word lines in memory array 352 aredriven by row decoder 349. Column decoders 344 and 346 may be split suchthat even bit lines in memory array 352 are controlled by column decoder346 and odd bit lines in memory array 352 are driven by column decoder344. The selected bit lines controlled by column decoder 344 may beelectrically coupled to read/write circuits 340. The selected bit linescontrolled by column decoder 346 may be electrically coupled toread/write circuits 342. Splitting the read/write circuits intoread/write circuits 340 and 342 when the column decoders are split mayallow for a more efficient layout of the memory bay.

In one embodiment, the memory arrays 352 and 354 may comprise memorylayers that are oriented in a horizontal plane that is horizontal to thesupporting substrate. In another embodiment, the memory arrays 352 and354 may comprise memory layers that are oriented in a vertical planethat is vertical with respect to the supporting substrate (i.e., thevertical plane is perpendicular to the supporting substrate). In thiscase, the bit lines of the memory arrays may comprise vertical bitlines.

FIG. 2 depicts one embodiment of a portion of a monolithicthree-dimensional memory array 201 that includes a second memory level220 positioned above a first memory level 218. Memory array 201 is oneexample of an implementation for memory array 301 in FIG. 1E. The bitlines 206 and 210 are arranged in a first direction and the word lines208 are arranged in a second direction perpendicular to the firstdirection. FIG. 2 is an example of a horizontal cross-point structure inwhich word lines 208 and bit lines 206 and 210 both run in a horizontaldirection relative to the substrate, while the memory cells 200 areoriented so that the current runs in the vertical direction. Asdepicted, the upper conductors of first memory level 218 may be used asthe lower conductors of the second memory level 220 that is positionedabove the first memory level. In a memory array with additional layersof memory cells, there would be corresponding additional layers of bitlines and word lines.

As depicted in FIG. 2, memory array 201 includes a plurality of memorycells 200. The memory cells 200 may include re-writeable memory cells.The memory cells 200 may include non-volatile memory cells or volatilememory cells. With respect to first memory level 218, a first portion ofmemory cells 200 are between and connect to bit lines 206 and word lines208. With respect to second memory level 220, a second portion of memorycells 200 are between and connect to bit lines 210 and word lines 208.In one embodiment, each memory cell includes a select switch or steeringelement (e.g., an ovonic threshold switch or a diode) and a memoryelement (i.e., a phase change memory element). In one example, thecurrent in the memory cells of the first memory level 218 may flowupward as indicated by arrow A₁, while the current flowing in memorycells of the second memory level 220 may flow downward as indicated byarrow A₂, or vice versa. In another embodiment, each memory cellincludes a state change element and does not include a select switchelement. The absence of a select switch (or other steering element) froma memory cell may reduce the process complexity and costs associatedwith manufacturing a memory array.

In one embodiment, the memory cells 200 of FIG. 2 may comprisere-writable non-volatile memory cells, including a reversibleresistance-switching element such as the phase change memory (PCM)embodiments described in more detail below. A reversibleresistance-switching element may include a reversibleresistivity-switching material having a resistivity that may bereversibly switched between two or more states.

Referring to FIG. 2, in one embodiment of a read operation, such ascould be used when a memory cell is a PCM material in series withpolarity asymmetric element (e.g., a diode), the data stored in one ofthe plurality of memory cells 200 may be read by biasing one of the wordlines (i.e., the selected word line) to a selected word line voltage inread mode (e.g., 0V). A read circuit may then be used to bias a selectedbit line connected to the selected memory cell to the selected bit linevoltage in read mode (e.g., 1.0V). In some cases, in order to avoidsensing leakage current from the many unselected word lines to theselected bit line, the unselected word lines may be biased to the samevoltage as the selected bit lines (e.g., 1.0V). To avoid leakage currentfrom the selected word line to the unselected bit lines, the unselectedbit lines may be biased to the same voltage as the selected word line(e.g., 0V); however, biasing the unselected word lines to the samevoltage as the selected bit lines and biasing the unselected bit linesto the same voltage as the selected word line may place a substantialvoltage stress across the unselected memory cells driven by both theunselected word lines and the unselected bit lines.

In an alternative read biasing scheme, such as could be used when amemory cell is a PCM material in series with an ovonic threshold switch(as discussed further below), both the unselected word lines and theunselected bit lines may be biased to an intermediate voltage that isbetween the selected word line voltage and the selected bit linevoltage. Applying the same voltage to both the unselected word lines andthe unselected bit lines may reduce the voltage stress across theunselected memory cells driven by both the unselected word lines and theunselected bit lines; however, the reduced voltage stress comes at theexpense of increased leakage currents associated with the selected wordline and the selected bit line. Before the selected word line voltagehas been applied to the selected word line, the selected bit linevoltage may be applied to the selected bit line, and (in an off-currentcancelation scheme) a read circuit may then sense an auto zero amount ofcurrent through the selected memory bit line which is subtracted fromthe bit line current in a second current sensing when the selected wordline voltage is applied to the selected word line. Embodiments for thereading of multi-state data in phase change memory are described in moredetail below with respect to FIGS. 10A-10C.

Referring to FIG. 2, in one embodiment of a write operation, thereversible resistance-switching material may be in an initialhigh-resistivity state that is switchable to a low-resistivity stateupon application of a first voltage and/or current. Application of asecond voltage and/or current may return the reversibleresistance-switching material back to the high-resistivity state.Alternatively, the reversible resistance-switching material may be in aninitial low-resistance state that is reversibly switchable to ahigh-resistance state upon application of the appropriate voltage(s)and/or current(s). When used in a memory cell, one resistance state mayrepresent a binary data “0” while another resistance state may representa binary data “1.” In some cases, a memory cell may be considered tocomprise more than two data/resistance states (i.e., a multi-level ormulti-state memory cell). In some cases, a write operation may besimilar to a read operation except with a larger voltage range placedacross the selected memory cells. Embodiments for the writing ofmulti-state data in phase change memory is described in more detailbelow with respect to FIGS. 9A-9E.

The process of switching the resistance of a reversibleresistance-switching element from a high-resistivity state to alow-resistivity state may be referred to as SETTING the reversibleresistance-switching element. The process of switching the resistancefrom the low-resistivity state to the high-resistivity state may bereferred to as RESETTING the reversible resistance-switching element.The high-resistivity state may be associated with binary data “1” andthe low-resistivity state may be associated with binary data “0.” Inother embodiments, SETTING and RESETTING operations and/or the dataencoding may be reversed. For example, the high-resistivity state may beassociated with binary data “0” and the low-resistivity state may beassociated with binary data “1.” In some embodiments, a higher thannormal programming voltage may be required the first time a reversibleresistance-switching element is SET into the low-resistivity state asthe reversible resistance-switching element may have been placed into aresistance state that is higher than the high-resistivity state whenfabricated. The term “FORMING” may refer to the setting of a reversibleresistance-switching element into a low-resistivity state for the firsttime after fabrication or the resetting of a reversibleresistance-switching element into a high-resistivity state for the firsttime after fabrication. In some cases, after a FORMING operation or amemory cell preconditioning operation has been performed, the reversibleresistance-switching element may be RESET to the high-resistivity stateand then SET again to the low-resistivity state.

Referring to FIG. 2, in one embodiment of a write operation, data may bewritten to one of the plurality of memory cells 200 by biasing one ofthe word lines (i.e., the selected word line) to the selected word linevoltage in write mode (e.g., 5V). A write circuit may be used to biasthe bit line connected to the selected memory cell to the selected bitline voltage in write mode (e.g., 0V). In some cases, in order toprevent program disturb of unselected memory cells sharing the selectedword line, the unselected bit lines may be biased such that a firstvoltage difference between the selected word line voltage and theunselected bit line voltage is less than a first disturb threshold. Toprevent program disturb of unselected memory cells sharing the selectedbit line, the unselected word lines may be biased such that a secondvoltage difference between the unselected word line voltage and theselected bit line voltage is less than a second disturb threshold. Thefirst disturb threshold and the second disturb threshold may bedifferent depending on the amount of time in which the unselected memorycells susceptible to disturb are stressed.

In one write biasing scheme, both the unselected word lines and theunselected bit lines may be biased to an intermediate voltage that isbetween the selected word line voltage and the selected bit linevoltage. The intermediate voltage may be generated such that a firstvoltage difference across unselected memory cells sharing a selectedword line is greater than a second voltage difference across otherunselected memory cells sharing a selected bit line. One reason forplacing the larger voltage difference across the unselected memory cellssharing a selected word line is that the memory cells sharing theselected word line may be verified immediately after a write operationin order to detect a write disturb.

FIG. 3 depicts a subset of the memory array and routing layers of oneembodiment of a three-dimensional memory array, such as memory array 301in FIG. 1E. As depicted, the Memory Array layers are positioned abovethe Substrate. The Memory Array layers include bit line layers BL0, BL1and BL2, and word line layers WL0 and WL1. In other embodiments,additional bit line and word line layers can also be implemented.Supporting circuitry (e.g., row decoders, column decoders, andread/write circuits) may be arranged on the surface of the Substratewith the Memory Array layers fabricated above the supporting circuitry.An integrated circuit implementing a three-dimensional memory array mayalso include multiple metal layers for routing signals between differentcomponents of the supporting circuitry, and between the supportingcircuitry and the bit lines and word lines of the memory array. Theserouting layers can be arranged above the supporting circuitry that isimplemented on the surface of the Substrate and below the Memory Arraylayers.

As depicted in FIG. 3, two metal layers R1 and R2 may be used forrouting layers; however, other embodiments can include more or less thantwo metal layers. In one example, these metal layers R1 and R2 may beformed of tungsten (about 1 ohm/square). Positioned above the MemoryArray layers may be one or more top metal layers used for routingsignals between different components of the integrated circuit, such asthe Top Metal layer. In one example, the Top Metal layer is formed ofcopper or aluminum (about 0.05 ohms/square), which may provide a smallerresistance per unit area than metal layers R1 and R2. In some cases,metal layers R1 and R2 may not be implemented using the same materialsas those used for the Top Metal layers because the metal used for R1 andR2 must be able to withstand the processing steps for fabricating theMemory Array layers on top of R1 and R2 (e.g., satisfying a particularthermal budget during fabrication).

FIGS. 2 and 3 illustrate a horizontal cross-point architecture, wherebit lines and word lines run horizontally with respect to the substrate.Although the memory cell structure described below, where a PCM materialis confined within a pocket, or pore, structure lined with a surfactantcan be used in a horizontal cross-point arrangement, most of thefollowing discussion is in the context of vertical cross-pointstructure. In a three-dimensional vertical cross-point structure, theword lines still run horizontally, but the bit lines are now oriented torun in a vertical direction. FIG. 4 illustrates a vertical cross-pointstructure.

FIG. 4 depicts one embodiment of a portion of a monolithicthree-dimensional memory array 416 that includes a first memory level412 positioned below a second memory level 410. Memory array 416 is oneexample of an implementation for memory array 301 in FIG. 1E. Asdepicted, the local bit lines LBL₁₁-LBL₃₃ are arranged in a firstdirection (i.e., a vertical direction) and the word lines WL₁₀-WL₂₃ arearranged in a second direction perpendicular to the first direction.This arrangement of vertical bit lines in a monolithic three-dimensionalmemory array is one embodiment of a vertical bit line memory array. Asdepicted, disposed between the intersection of each local bit line andeach word line is a particular memory cell (e.g., memory cell M₁₁₁ isdisposed between local bit line LBL₁₁ and word line WL₁₀). Thisstructure can be used with a number of different memory cell structures.In one example, the particular memory cell may include a floating gatedevice or a charge trap device (e.g., using a silicon nitride material).In another example, the particular memory cell may include a reversibleresistance-switching material, a metal oxide, a phase change memorymaterial, or a ReRAM material. The discussion in the following focusseson the use of a PCM material. The global bit lines GBL₁-GBL₃ arearranged in a third direction that is perpendicular to both the firstdirection and the second direction. A set of bit line select devices(e.g., Q₁₁-Q₃₁), such as a vertical thin film transistor (VTFT), may beused to select a set of local bit lines (e.g., LBL₁₁-LBL₃₁). Asdepicted, bit line select devices Q₁₁-Q₃₁ are used to select the localbit lines LBL₁₁-LBL₃₁ and to connect the local bit lines LBL₁₁-LBL₃₁ tothe global bit lines GBL₁-GBL₃ using row select line SG₁. Similarly, bitline select devices Q₁₂-Q₃₂ are used to selectively connect the localbit lines LBL₁₂-LBL₃₂ to the global bit lines GBL₁-GBL₃ using row selectline SG₂ and bit line select devices Q₁₃-Q₃₃ are used to selectivelyconnect the local bit lines LBL₁₃-LBL₃₃ to the global bit linesGBL₁-GBL₃ using row select line SG₃. In the examples discussed belowwith respect to FIG. 7A, the select devices Q₁₁-Q₃₃ are implemented asvertical thin film transistors, but other embodiments can be used.

Referring to FIG. 4, as only a single bit line select device is used perlocal bit line, only the voltage of a particular global bit line may beapplied to a corresponding local bit line. Therefore, when a first setof local bit lines (e.g., LBL₁₁-LBL₃₁) is biased to the global bit linesGBL₁-GBL₃, the other local bit lines (e.g., LBL₁₂-LBL₃₂ and LBL₁₁-LBL₃₃)must either also be driven to the same global bit lines GBL₁-GBL₃ or befloated. In one embodiment, during a memory operation, all local bitlines within the memory array are first biased to an unselected bit linevoltage by connecting each of the global bit lines to one or more localbit lines. After the local bit lines are biased to the unselected bitline voltage, then only a first set of local bit lines LBL₁₁-LBL₃₁ arebiased to one or more selected bit line voltages via the global bitlines GBL₁-GBL₃, while the other local bit lines (e.g., LBL₁₂-LBL₃₂ andLBL₁₃-LBL₃₃) are floated. The one or more selected bit line voltages maycorrespond with, for example, one or more read voltages during a readoperation or one or more programming voltages during a programmingoperation.

FIG. 5 depicts one embodiment of a read/write circuit 502 along with aportion of a memory array 501. Read/write circuit 502 is one example ofan implementation of read/write circuit 306 in FIG. 1D. The portion of amemory array 501 includes two of the many bit lines (one selected bitline labeled “Selected BL” and one unselected bit line labeled“Unselected BL”) and two of the many word lines (one selected word linelabeled “Selected WL” and one unselected word line labeled “UnselectedWL”). The portion of a memory array also includes a selected memory cell550 and unselected memory cells 552-556. In one embodiment, the portionof a memory array 501 may comprise a memory array with bit linesarranged in a direction horizontal to the substrate, such as memoryarray 201 in FIG. 2. In another embodiment, the portion of a memoryarray 501 may comprise a memory array with bit lines arranged in avertical direction that is perpendicular to the substrate, such asmemory array 416 in FIG. 4.

The memory array 501 may include a plurality of phase change memorycells. In one embodiment, each of the memory cells within the memoryarray 501 may include a PCM material in series with an ovonic thresholdswitch, where the ovonic threshold switch would be located between thePCM material and either the bit line or word line. In one embodiment,the PCM material may comprise a chalcogenide material like Ge₂Sb₂Te₅(GST).

As depicted, during a memory array operation (e.g., a programmingoperation), the selected bit line may be biased to 1V, the unselectedword line may be biased to 0.6V, the selected word line may be biased to0V, and the unselected bit line may be biased to 0.5V. In someembodiments, during a second memory array operation, the selected bitline may be biased to a selected bit line voltage (e.g., 2.0V), theunselected word line may be biased to an unselected word line voltage(e.g., 1.0V), the selected word line may be biased to a selected wordline voltage (e.g., 0V), and the unselected bit line may be biased to anunselected bit line voltage (e.g., 1V). In this case, the unselectedmemory cells sharing the selected word line will be biased to thevoltage difference between the selected word line voltage and theunselected bit line voltage. In other embodiments, the memory arraybiasing scheme depicted in FIG. 5 may be reversed such that the selectedbit line is biased to 0V, the unselected word line is biased to 0.4V,the selected word line is biased to 1V, and the unselected bit line isbiased to 0.5V.

As depicted in FIG. 5, the SELB node of read/write circuit 502 may beelectrically coupled to the selected bit line via column decoder 504. Inone embodiment, column decoder 504 may correspond with column decoder302 depicted in FIG. 1E. The write circuit 560 and the read circuit 566couples (or electrically connects) to the column decoder 504 through thenode SELB for sensing the data stored in a coupled memory cell. Theoutput of the read circuit 566 is connected to the data out terminal andto data latch 568. Write circuit 560 is connected to node SELB, the DataIn terminal, and data latch 568.

In one embodiment, during a read operation, read/write circuit 502biases the selected bit line to the selected bit line voltage in readmode. If the current through the selected memory cell 550 is greaterthan the read current limit, Iref, then, over time, the SELB node willfall below Vref-read (e.g., set to 1.5V) and the read circuit 566 willread out a data “0.” Outputting a data “0” represents that the selectedmemory cell 550 is in a low resistance state (e.g., a SET state). If thecurrent through the selected memory cell 550 is less than Iref, then theVsense node will stay above Vref-read and the read circuit 566 will readout a data “1.” Outputting a data “1” represents that the selectedmemory cell 550 is in a high resistance state (e.g., a RESET state).Data latch 568 may latch the output of read circuit 566 after a timeperiod of sensing the current through the selected memory cell (e.g.,after 400 ns). More detail on an embodiment for the read circuit 566 andits operation for a multi-state phase change memory is given below withrespect to FIGS. 10A-10C, after the discussion of the memory cellstructure.

In one embodiment, during a write operation, if the Data In terminalrequests a data “0” to be written to a selected memory cell, thenread/write circuit 502 may bias SELB to the selected bit line voltagefor programming a data “0” in write mode via write circuit 560. Theduration of programming the memory cell may be a fixed time period(e.g., using a fixed-width programming pulse) or variable (e.g., using awrite circuit 560 that senses whether a memory cell has been programmedwhile programming). If the Data In terminal requests a data “1” or othernon “0” value to be written, then read/write circuit 502 may bias SELBto the selected bit line according via write circuit 560. In some cases,if a selected memory cell is to maintain its current state, then thewrite circuit 560 may bias SELB to a program inhibit voltage duringwrite mode. The program inhibit voltage may be the same as or close tothe unselected bit line voltage. In the following, the erased state istaken as the SET, or the low resistance crystalline state, with otherstates being RESET or partially RESET, corresponding the various degreesof the PCM material being in the amorphous state. More detail on anembodiment for the write circuit 560 and its operation for a multi-statephase change memory is given below with respect to FIGS. 9A-9E, wherememory cells are written from the crystalline SET state to the differenttarget states by applying current pulses of different amplitudes thatraise the PCM material (or a portion of it) above the melt temperature.An erase operation to take a memory cell back to the SET crystallinestate is effected by performing a write operation where the currentpulse is sufficient to heat the PCM material above its crystallizationtemperature, but not above its melt temperature.

One way to increase memory density is through the stacking of multiplememory layers, as illustrated above for horizontal cross-point, as inFIGS. 2 and 3 where both of the bit lines and word lines run in ahorizontal direction, or for a vertical cross-point structure, as inFIG. 4 where the word lines run in horizontal direction and the bit linerun in a vertical direction. A complimentary method for increasingmemory density is through use of multi-state, or multi-level, memorycells (MLCs), where each cell stores more than one bit of data. Thefollowing considers a memory structure the combines multiple memorylayers in a vertical cross-point structure with multi-state memory cellcapability for a phase change memory.

A memory cell formed from a phase change memory material, such as achalcogenide material like Ge₂Sb₂Te₅ (GST), may change phases between anamorphous state (e.g., corresponding with a high resistance reset state)and a crystalline state (e.g., corresponding with a low resistance setstate) or between a less-ordered crystalline state and a more-orderedcrystalline state. Based upon the portion of a memory cell that is anamorphous state relative to the crystalline state, a phase change memorycell can be considered an “analog” device that can be programmed intoone of a multitude of states for MLC operation. By appropriatemodulation of programming current/voltage, this can result in acontinuous modulation of PCM resistive states, allowing PCM to be usedas multi-state cells. To accurately store multi-state data, however, theprogrammed states of the cells need to be, and remain, stable, so thatthe resistive state of a cell does not exhibit an excessive amount ofdrift/change over time or operating conditions (such as temperature).

FIGS. 6A-C illustrate a PCM memory cell with multiple resistance levels.In each of FIGS. 6A-C, a phase change memory material 603 is connectedbetween a word line electrode 601 and a bit line electrode 605. (Thelayer S 607 is discussed below.) In FIG. 6A, all of the PCM material isin the crystalline, lower resistivity (or SET) state, as indicated bythe letter “C”. In FIG. 6B, a higher resistivity amorphous (or RESET)region marked by the letter “A” is formed in the memory cell's centralregion, with a larger amorphous region “A” illustrated in the memorycell of FIG. 6C.

The resistance of the different memory cell states is modelled by thecorresponding circuit diagram above each memory cell representation(FIGS. 6A-6C), with the lower resistance C regions represented by thelighter line weight resistors (non-bolded) and the higher resistance Aregions represented by the heavier line weight resistors (bolded).Ignoring for a moment the resistances marked “S”, by having anincreasing portion the PCM material in an amorphous state, the memorycells of FIGS. 6A, 6B and 6C will have increasing resistance levels.(Although not shown in FIGS. 6A-6C, the parallel current paths of thePCM material 603 and the surfactant 607 can connected in series withselector, such as an ovonic threshold switch, as discussed below.) Eachof the resistance levels corresponds to a different data state. Moredata states can similarly be introduced by a finer control of theamorphous to crystalline proportions of the memory cell.

The layer S 607 in parallel with the phase change memory material 603between the word line electrode 601 and the bit line electrode 605represents a surfactant, such as a metal-nitride layer, that can beintroduced to increase the adhesion of the PCM material to the otherlayers and suppress drift the memory cell states. To maintain dataintegrity, the data states stored on a memory cell should be relativelystable over time and varying operating conditions, such as temperature.The surfactant layer can provide this stability such that more states tobe accurately stored in a PCM memory cell. The surfactant layer 607 isarranged, or configured, to have a resistivity intermediate to thecrystalline state and the amorphous state of the PMC material, asrepresent by the parallel chain of resistances labelled “S” of anintermediate line weight.

In one embodiment, the surfactant layer 607 has a resistance greaterthan the crystalline state of the PMC material and less than theamorphous state of the PMC material. The primary read path for currentin each of FIGS. 6A-C is represented by the arrows, corresponding tothree different resistance states. The resistivity of the amorphousregion can drift over time and operating conditions, so that if thecurrent path between the word line and bit line runs through amorphousregion, this can make the accurate storage of multi-state data difficultin the memory cell. The crystalline phase of the PCM material and thesurfactant both have relatively stable resistivities. By having asurfactant layer with a resistance intermediate to that of thecrystalline and amorphous phase regions of the PCM material adjacent toand in contact with the PCM material, this provides a second currentpath between the word line and bit line. This allows for the currentbetween the word line and bit line to bypass the relatively unstableamorphous region, but still increase the resistance level by increasingthe proportion of the PCM material in the amorphous state. The use of asurfactant with the phase change memory cell can improve multi-stateoperation of such memory cells, whether in a horizontal cross-pointmemory structure, as in FIGS. 2 and 3, in a vertical cross-pointstructure, as in FIG. 4, or in other structures using such memory cells.

More specifically, the incorporation of a multi-state PCM memory cellwithin a vertical cross point architecture can provide a high-densitymemory structure. The use of the surfactant, such as a metal-nitride, ina recessed word line structure for GST or other PCM material, can aidwith adhesion and drift suppression. FIG. 7A illustrates such astructure, according to one embodiment.

FIG. 7A shows cross-sectional side view one embodiment of a verticalcross-point memory structure with recessed PCM memory cells taken fromthe line indicated in FIG. 7B. FIG. 7A illustrates a side view of aportion of a vertical cross-point array such as described above withrespect to FIG. 4, illustrating a single local vertical bit line 721,such as LBL₁₁ in FIG. 4, and the word lines (such as 711) to eitherside. Relative to the embodiments described in FIG. 4, the phase changememory material (PCM 701) of the non-volatile memory cell in theembodiment of FIG. 7A is confined in a pore-like recessed pocket regionof the word lines, as described below. The array using such a verticalcross-point structure with the confined, recessed PCM material can thenbe incorporated in a memory device as illustrated with respect to FIGS.1A-1F

In the embodiment illustrated in FIG. 7A, under a top dielectric layer712 a series of word lines WL (e.g. 711) alternate with dielectriclayers D (e.g. 713). The example of FIG. 7A shows four word line layers,but other embodiments can use a different number of word lines. A localvertical bit line VBL 721 is connected to a global bit line GBL 733through a select switch 731, in this example a vertical thin filmtransistor VTFT, but other switches can be used. The VTFT includes acentral P− region, with N+ regions above and below and a control gate tothe sides. The lower dielectric layer 714 and global bit lines such asGBL 733 are formed over a substrate (not shown). In some embodiments,the region under the lower dielectric layer and global bits lines canincludes CMOS or other circuit elements (decoders, drivers, and so on)for operation of the array. The vertical bit line VBL 721 is formed inthe trench or memory hole structure between the word line layers. In theexample of FIG. 7A, the word lines on either side of the bit line areseparate, but in other embodiments, the word lines on either side may beconnected and surround VBL 721 on one or both sides.

Each word line WL is recessed from the vertical bit line VBL 721relative the inter-word line dielectric layers D, forming a set ofrecessed pockets within which the phase change memory material PCM 701is located. The thickness of the vertical dimension of the recessedpocket is based on the word line thickness and can be, for example, onthe order of 5 nm or somewhat larger, while the width (the distance thatit extends toward the vertical bit line) of the recessed pocket may be,for example, 10 nm or more. In the example, the memory cells to the leftand to the right of VBL 721 are distinct, but in other embodiments canbe portions of a common memory cell surrounding the VBL 721 to thefront, behind or both sides. The example of FIG. 7A also includes asurfactant layer (such as 703), such as a metal-nitride, lining therecessed pocket within which the GST or other PCM 701 is located, wherethe surfactant can aid with adhesion and drift suppression to improvemulti-state operation as described with respect to FIGS. 6A-6C. In theembodiment of FIG. 7A, in addition to being around (to the sides, aboveand below) the PCM material 701 to provide a parallel current pathbetween the word lines and the bit lines, the surfactant 703 is also ina thin layer between the PCM material 701 and the word line 711.Although not needed for providing the parallel current path, this is aresult in some embodiments of forming the surfactant lining of thepocket or pore structure within which the PCM material will later beformed. Alternate embodiments can omit the surfactant between the wordline layers WL and the PCM material and/or some of the surfactantseparating the PCM material from the dielectric, but still provide thesurfactant on one or more sides (for example, lining the pocket justalong the bottom) to provide the adjacent, parallel current pathdescribed with respect to FIG. 6A-6C. Still other embodiments may omitthe surfactant, operate the memory cell as a binary device, or implementa combination of both variations.

The phase change memory material PCM 701 of each memory cell isseparated from the vertical bit line VBL 721 by selector switch, such asby an ovonic threshold switch layer OTS 723. The ovonic threshold switchlayer OTS 723 is made of a material, such as glass of specialcomposition, with the ability to change from an electricallynonconducting state to a semiconducting state upon application of acertain minimum voltage differential. OTS materials are usuallyamorphous semiconductors covering a wide range of compositions. Thesecan include oxide-, boron- and chalcogenide-based glasses combined withother elements of groups III, IV, V and VI. When a high enough voltagedifference is applied between the vertical bit line VBL 721 and the wordline WL 711 relative to the resistance state of the PCM material 701,the OTS 723 will act as a conductor providing a current path between theword line WL 711 to the bit line VBL 721. A carbon C 725 or other layerwith an electric resistivity higher than the word lines and bit lineslayers can be formed between the OTS 723 and PCM 701 if needed toprovide separation to avoid intermixing of the material during operationof the memory cell. In other embodiments, the carbon or other diffusionbarrier layer may be omitted if the OTS material is stable enough to notintermix with the PCM material during operation.

FIG. 8 is an equivalent circuit diagram representation of the portion ofthe memory structure illustrated in FIG. 7A. As described with respectto FIGS. 6A-6C, each PCM memory cell is a variable resistance devicehaving a value dependent upon the state to which it is programmed. EachPCM memory cell is connected between a word line WL (such as 711) and abit line VBL (such as 721). The local bit line VBL 721 is connected tothe global bit line 733 through a switch 731, whether a vertical thinfilm transistor VTFT or other type of switch, controlled by the level ofan LBL voltage on the control gate of the VTFT 731. The PCM memory cellsare each connected to the bit line VBL 721 through the ovonic thresholdswitch layer OTS 723 or other selector switch, which acts as a seriesconnected switching element with the PCM memory cell, that will conductwhen a sufficient voltage difference is applied across it. Within alarger array structure, this corresponds to a more detailedrepresentation of the memory cells along either side of a vertical bitline, such as LBL₁₁ of FIG. 4, where the memory cells of FIG. 4correspond to the series combination of the phase change memory materialPCM in series with the ovonic threshold switch OTS. Other examples ofselector switches can include polysilicon PN junctions, oxide PNjunctions, oxide rectifiers (Pt/TiO₂/Ti), or mixed ion-electronconductor (MIEC), among others.

In FIG. 7A, current can flow into the PCM material 701 throughsurfactant layer 703 from the corresponding word line 711. If thedielectric layer D between adjacent word lines is sufficiently thick,significant current will not flow from the either the word line above orbelow a selected memory cell. If the sides (the directions into and outthe page) of a memory cell are in contact to the conductive word linelayer WL, current may flow in to the memory cells from the sides, makingmulti-state operation more challenging. The can be explained further byreference to FIGS. 7B-7D.

FIG. 7B is top view of one embodiment of a vertical cross-point memorystructure with PCM memory cells confined within a recess. In this FIG.7B, the view is looking down at a word line layer on a number ofvertical bit line structures with distinct PCM memory cells above andbelow (as represented in FIG. 7B) each vertical bit line (VBL). Across-sectional cut that corresponds to FIG. 7A is indicated for one ofthe vertical bit line structures. In this embodiment, word lines WL1,WL2, WL3, WL4 run horizontally and are connected by vertical sections toform two U shapes. For each vertical bit line, a memory cell isconnected to a word line on either side of the vertical bit line. Theembodiment of FIG. 7B again includes the surfactant and the carbon layerC.

In FIG. 7B each of the memory cells is located in a pocket, formed byremoving a prong extending from a word line towards bit line. Forexample, the PCM material 701 is formed with the pocket lined with thesurfactant 703. The area to the sides of memory cells, separating thePCM material of one memory cell for an adjacent memory cell to eitherside along the same word line is filled with dielectric D of layer 713.The dielectric causes the current flowing from the word line 711 to thememory cell to enter from the “back” of a cell, where this desiredcurrent flow is illustrated in FIG. 7C.

In an alternate set of embodiments, the pocket for a memory cell can berecessed into the word line, as illustrated FIG. 7D. The embodiment ofFIG. 7D can be used to store a single bit per cell. However, such anarchitecture is less desirable for storing multiple bits per cell. Asillustrated in FIG. 7D, in this arrangement, current can also enter thememory cell from the sides. For multi-state/multi-bit per celloperation, this current is typically unwanted current flow as it canmake multi-state programming and reading less controllable as thecurrent will concentrate nearer to the bit line. The confined (or “pore”type) memory cell structure of FIG. 7C in which the PCM material isconfined by dielectric on all but one side, combined with the surfactantpocket lining make the current levels more controllable, allowing forlower power and multi-state operation. A confined memory cell structurepositioned within the pocket, as in FIGS. 7B and 7C provides thermalconfinement such that lower programming current can be used to operatethe PCM material of the memory cell.

To avoid this unwanted side current, rather forming the memory cells inpockets within the word lines (as in FIG. 7D), the word line layersinstead use a “crested”, or pronged, structure, where the pockets forthe PCM material of the memory cells are formed within the areapreviously occupied by prongs extending outward from the word linestowards each of the vertical bit line structures. This is the structureillustrated in FIGS. 7B and 7C, where the region between adjacent memorycells is filled with a dielectric material. The PCM material andsurrounding surfactant of each memory cell is then formed within onethese word line pockets formed within the prongs. Whether the “crested”word line structure with these prongs as in FIGS. 7B and 7C, or in thesort of structure illustrated in FIG. 7D where the memory cells pocketsare recessed with word line material to either side, the cross-sectionalview of FIG. 7A would be the same.

To write multi-state data to a PCM memory cell, the memory cell caneither start in the low resistance crystalline stage as in FIG. 6A andhave the relative amount of amorphous region increased as in in FIGS. 6Band 6C, or start in the amorphous state and have the crystalline regionincreased. In either case, the state of the PCM material can be changedby applying a current pulse, where to move to a more amorphous state thepulse will raise the PCM material above the melt temperature; and tomove to a more crystalline state the pulse will raise the PCM materialabove the crystallization temperature, but not above the melttemperature. In the embodiments discussed below, the erased state willbe taken as the low resistance crystalline state and the higherprogrammed states will be taken as have increasing levels of the higherresistance from a larger amorphous region, but in other embodimentsthese could be reversed. FIGS. 9A-9E illustrate an embodiment for awrite circuitry and its operation, which can also be used in an eraseoperation as in either case a current pulse is used to change the stateof at least a portion of the PCM material.

FIG. 9A shows one embodiment for write circuitry such as can beincorporated in the write circuit 560 of FIG. 5 for the PCM memorystructure described with respect to FIGS. 1A-8. FIG. 9A shows a portionof the write circuitry connected to a single selected memory cell 900.Decoding and select circuitry, such as column decoder 504 in FIG. 5, andother elements are omitted in FIG. 9A to simplify the discussion. Thewrite circuit 560 of FIG. 5 would typically have multiple copies of thewrite circuitry elements illustrated in FIG. 9A connected throughdecoding circuitry to be able to concurrently program multiple memorycells.

The selected memory cell 900 in this embodiment includes a phase changememory material PCM 901 connected in series with an ovonic thresholdswitch OTS 903 between a bit line BL 905 and word line WL 907. The stateof the PCM material 901 is changed by applying a current I_(prog)through the cell 901. In the shown embodiment, the word line 907 isplaced at a low voltage level, such as ground, and the programmingcurrent I_(prog) is applied from the bit line BL 905. The programmingcurrent I_(prog) is provided by the transistor 915 that is connectedbetween the bit line BL905 and a high voltage level, where this can be aregulated supply level on the memory circuity or a raised level such ascan be generated by a charge pump.

The transistor 915 is part of current mirror, mirroring the currentthrough transistor 913, which is connected between the high and lowvoltage levels in series with a current source 911 providing I_(prog).In this embodiment, the transistors 913 and 915 are here implemented asPMOS devices, but other embodiments can use N type devices. In asymmetric current mirror, where transistors are of the samewidth/length, I_(prog) will be the same as I_(prog), but differentratios can be obtained by varying the relative device sizes. Differentvalues for I_(prog) can be obtained by varying I_(prog), by havingmultiple transistors connected similarly to 915 of varying dimensionsmirroring the I_(prog) value by differing ratios, or some combination ofthese. Additional selected memory cells can simultaneously be written byrepeating the current mirror arrangement, mirroring the current inmultiple transistors, or some combination of these.

In an embodiment where the memory cells are written by starting with thePCM material in a crystalline state (as in FIG. 6A) and increasing theamount of the amorphous region (as in FIGS. 6B and 6C), differentprogrammed states can be achieved by using different I_(prog) levels.This illustrated in FIG. 9B.

FIG. 9B illustrates several write pulses for the programming currentI_(prog) that can be used in one embodiment to program a PCM memory cellin a reset, or partial reset, operation. Starting initially with the PCMin a crystalline state, three pulse amplitude are shown, correspondingto three different levels of programming (increased amorphous region)from the crystalline state. The current increases the temperature in thememory cell, melting the PCM's crystal region. The melt current scaleswith memory cell size, with, for example, a current of around 100 μA fora 20 nm memory cell. The program current can exceed the correspondingmelt current, while staying within a level that maintains cell integrityand keeps adjacent memory cells within their thermal disturb limits.FIG. 9B shows three different current levels 931, 933 and 935, whichwill respectively melt increasingly larger regions of the PCMcrystalline region. A few nanoseconds or tens of nanoseconds above themelt temperature can be sufficient to melt the PCM material withoutdegrading cell integrity or disturbing adjacent memory cells. A rapidtermination (a few nanoseconds) of the current pulse drops thetemperature, locking in the solid amorphous PCM structure. For example,the pulses 921 and 923 could correspond to the respective states shownin FIGS. 6B and 6C, with pulse 925 corresponding to an even largeramorphous region.

Depending on the embodiment, data programming to different states canuse current of differing amplitudes (as in FIG. 9B), differingdurations, or a combination of these. A single-pulse algorithm can beused, or multiple pulses, which may also include a verification betweensome or all of the pulses. Additionally, as the memory structure allowsfor the memory cells to be set and reset individually, if a memory cellis already in the target state (or in a lower state), it need not beerased beforehand to the crystalline state but left in its current stateor programmed without being first set to the crystalline state. FIG. 9Cis a flow for one embodiment of a program operation.

FIG. 9C depicts a flowchart describing an example of an embodiment ofprocesses for writing an array of phase change memory cells in which thememory cells are written from a crystalline state into states havingvarying degrees of the amorphous state PCM material. At step 920 thememory device receives form the controller the multi-state data to beprogrammed into memory cells along a word line. Depending on theembodiment, the memory cells along the selected word line may or may hadhave previously been erased (here, to the crystalline state of the PCMmaterial). As the structure described here allows for the memory cellsto be individually set and reset a number of variations are available,depending on the embodiment. In one set of embodiments, all of thememory cells along a word line can be, or previously will have been,erased. Alternately, the memory cells can selectively be erased, asdescribed with respect to step 921.

In some embodiments, if the memory cells along the word line have notalready been erased, a step 921 can be included to determine if any ofthe memory cells are in the desired state, or in a lower state, and arenot to be erased. The cells to be erased can then be erased. As thememory architecture allows for the PCM memory cells to be individuallyset and reset, in contrast to a flash memory structure where erase is atthe block level, in some embodiments only the memory cells needing to beerased can be erased. In other embodiments, the erase can be at wordline or other level. The erase process (setting of the PCM material tothe crystalline state) is discussed further with respect to FIGS. 9D and9E.

At step 922, the array is biased to select and deselect the memory cellsto be written by setting the word line and bit line levels accordingly.One embodiment for this is illustrated in FIG. 5, where the selectedword lines are set to ground, the non-selected word lines and bit linesat an intermediate voltage level, and the program selected bit lines setat a somewhat higher voltage. Under these bias conditions, the voltageacross the non-selected memory cells will not be sufficient to cause theovonic threshold switch to conduct, while the selected memory cells willconduct when a programming current pulse is applied in step 923. Theembodiments primarily discussed here bias a selected word line toground, allowing each selected bit line's pulse to be set separately.Alternate embodiments can apply the current pulses down the selectedword lines, but in this case, all of the cells along the selected wordline receive a common pulse.

In step 923 the programming pulse or pulses are applied to the selectedmemory cells. In one set of embodiments, these can be as illustrated inFIG. 9B, where, starting from the erased (crystalline) state, if amemory cell is to stay in the erased state or otherwise deselected, nopulse is applied; and if a memory cell is to be programmed, a currentpulse is applied with a state dependent amplitude. Depending on theembodiment, all memory cells along a word line or only a subset of thememory cells along the word line can be programmed concurrently.Additionally, in a multi-state embodiment all states can be programmedconcurrently or only a single state or subset can be programmed at atime. Also discussed with respect to FIG. 9B, rather than use a singlecurrent pulse whose amplitude depends on the target state, differenttarget states can alternately or additionally vary the pulse durationbased on the target state. In still other embodiments, multiple pulsecan be used, either with or without a verify/lock-out mechanism. Thechoice of programming algorithm can be based on the desired level ofaccuracy required in view of processing and operation parametervariations and the number of states being stored per memory cell.

As noted above, the PCM memory structures described here allow for thememory cells to be individually moved from the crystalline state to theamorphous state or from the amorphous state to the crystalline state. Inthe discussion of FIGS. 9A-9E, the crystalline state is taken as theerased state and programming to different states increases the size ofthe amorphous region to varying degrees (FIGS. 9A-9C), while the eraseprocess takes all of the memory cells' PCM material to the crystallinestate (FIGS. 9D and 9E, below). In an alternate set of embodiments, theroles can be reversed, where the amorphous state is taken as the erasedstate and the programming to different states increases the size of thecrystalline region to varying degrees. In this alternate set ofembodiments, the melting pulse of FIG. 9B would be sufficient to takeall selected cells to the amorphous state, while now the crystallizationpulses would be modulated to move the PCM material to the crystallinestate to varying degrees. In still other embodiments, memory cells couldbe written in either direction from the cell's current state to itstarget state. FIGS. 9D and 9E look at the amorphous to crystallinetransition pulse, where this corresponds to the erase operation of themain embodiments discussed here.

FIG. 9D illustrates one embodiment for a current pulse to set the memorycells' PCM to the crystalline phase, here taken as the erased state. Thecircuitry for applying the erase pulse can be the same as described withrespect to FIG. 9A, except that the current applied to the cell will nowbe sufficient to take the PCM material above its crystallizationtemperature, but not above its melt temperature. In the main embodimentsdiscussed here, as this is being used as an erase operation, the pulse(or cumulative effect of multiple pulse) should be sufficient to takeall memory cells to the sort of fully crystalline state illustrated inFIG. 6A.

In the embodiment illustrated in FIG. 9D, a single current pulse isapplied to the selected memory cells. The amplitude is sufficient toraise the PCM material above its crystallization temperature, but nothigh enough to reach the melt temperature. For example, the pulse couldbe of a duration of around 100-200 ns for a temperature in the 400°C.-200° C. to nucleate or grow the PCM crystal. In this example, acurrent of around 75 μA is shown. In alternate embodiments, a series ofpulses can be used, either with or without use of an eraseverify-lockout arrangement. In still other embodiments, rather than aconstant amplitude pulse, the bit line can be pre-charged and allowed todischarge so as to hold the PCM material in the crystallizationtemperature range for a sufficient amount of time.

FIG. 9E depicts a flowchart describing an example of an embodiment ofprocesses for erasing an array of phase change memory material basedmemory cells in which the memory cells are set to a crystalline state ofthe PCM. At step 940, the memory determines the memory cells to erase.As the PCM based memory structure described here allows for the memorycells to be set on an individual basis, the erase can be performed atdifferent levels of granularity: at the level of individual cells, allor part of an individual word line, or at the level of multiple wordlines, such as all of the word lines spanning a common set of bit lines,where the decision can be a tradeoff between performance and power.Additional, where a word line or other set of memory cells are selectedfor erase, memory cells already in the erased state may be de-selectedto save on power consumption. Depending on the embodiment, the eraseoperation can variously be performed in response to a specific erasecommand, or as part of a write command, such as before step 940 or aspart of step 941 in the flow of FIG. 9C.

At step 941, the array is biased to select, deselect the memory cellsfor erase. This can be implemented in much the same way as for step 922of FIG. 9C, as the difference will be in the subsequent pulse beingapplied at step 942 having a longer duration, but lower amplitude, thanthe pulses used in the programming process.

At step 942, the erase pulse, such as illustrated in the FIG. 9D, isapplied to the erased selected bit lines by the write circuity of FIG.9A. As discussed with respect to FIG. 9D, alternate embodiments can usemultiple pulses with or without an erase/lock-out mechanism. Otherembodiments can apply the erase pulse down selected word lines, ratherthan bit lines.

FIGS. 10A-10C consider the reading of multi-state memory cells for thePCM memory structure described with respect to FIG. 1A-8. FIG. 10Aillustrates the current-voltage characteristics of a PCM based memorycell for the PCM material alone, or PCM material and surfactant, and forwhen the ovonic threshold switch (OTS) is included. In this example,four states are shown, but other embodiments can have larger or smallernumbers of states per memory cell. At left are the curves for the PCMmaterial or PCM material and surfactant alone, without the OTS selectiondevice. At farthest left, the dashed line of trace 1020 is for acrystalline state, followed by the dashed line curves of three stateswith increasing levels of the PCM material in the amorphous state at1021, 1022 and 1023. The current increase linearly until the thresholdvoltage is reached, at which the cell is conductive, as shown atV_(th_PCM1), V_(th_PCM2) and V_(th_PCM1) for 1021, 1022 and 1023.

To the right in FIG. 10A, at left are the solid lines for the I-V curvesfor when the OTS material is now included, with 1030 corresponding towhen the PCM material in the memory cell is in crystalline (or set)state, followed by three states with increasing levels of the PCMmaterial in the amorphous state at 1031, 1032 and 1033. For each of1030-1033, the I-V curves initially increase linearly together untilthey separate as the OTS material of the memory cell become conducting.As the I-V curves 1030-1033 separate, they increase linearly until theyreach their respective threshold voltages, at which the cell isconductive. The threshold voltage for the crystalline (or set) phase isshown at V_(th_set) for 1030, with the threshold voltages for increasinglevels of amorphous (or reset) phase PCM material shown atV_(th_reset1), V_(th_reset2) and V_(th_reset1) for 1031, 1032 and 1033.The voltage levels V_(read1), V_(read2) and V_(read3) can be used todiscriminate between the memory cells' data states. If the programoperation of FIG. 9C uses multiple current pulses with inter-pulseverify operations, the voltage levels V_(read1), V_(read2) and V_(read3)or values somewhat offset from these levels can be used in verifysensing operations.

FIG. 10B shows one embodiment for read circuitry such as can beincorporated in the read circuit 566 of FIG. 5 for the PCM memorystructure described with respect to FIGS. 1A-8. FIG. 10B shows a portionof the write circuitry connected to a single selected memory cell 1000.Decoding and select circuitry, such as column decoder 504 in FIG. 5, andother elements are omitted in FIG. 10B to simplify the discussion. Theread circuit 566 of FIG. 5 would typically have multiple copies of thewrite circuitry elements illustrated in FIG. 9A connected throughdecoding circuitry to be able to concurrently read multiple memorycells.

The selected memory cell 1000 in this embodiment includes a phase changememory material PCM 1001 connected in series with an ovonic thresholdswitch OTS 1003 between a bit line BL 1005 and word line WL 1007. Thestate of the memory cell 1000 is read by applying a current I_(read)through the memory cell 1000. In the shown embodiment, the word line WL1007 is placed at a low voltage level, such as ground, and the currentI_(read) is applied from the read circuitry to the bit line BL 1005. Thecurrent I_(read) is provided by the transistor 1015 that is connectedbetween the bit line BL 1005 and a high voltage level, where this can bea regulated supply level on the memory circuity. The voltage on the gateof the transistor 1115 is applied by a driver 1117 as determined by theread voltage level V_(read) on the input of the driver 1117, whereV_(read) can be the voltage levels V_(read1), V_(read2) and V_(read3) ofFIG. 10A used to differentiate the memory cell data states illustratedin FIG. 10A. If V_(read) is above the threshold voltage of the memorycell, it will conduct resulting in a high I_(read) value, while ifV_(read) is below the threshold voltage of the memory cell, it will notconduct resulting in a low I_(read) value.

The current I_(read) through transistor 1015 is mirrored in transistor1013, which is connected between the high and low voltage levels inseries with a reference current source 1011 providing I_(ref). In thisembodiment, the transistors 1013 and 1015 are implemented as PMOSdevices. In a symmetric current mirror, where transistors are of thesame width/length, I_(read) will be the same in 1013 and 105, butdifferent ratios can be obtained by varying the relative device sizes.If I_(read) in transistor 1013 is higher than I_(ref), the sensingoutput voltage V_(read_out) will be high, while if the current I_(read)in transistor 1013 is lower than I_(ref), the sensing output voltageV_(read_out) will be low. The result of the read operation can be sentout as output data at Data Out for the read/write circuits 502 of FIG. 5or stored in the data latch 568.

FIG. 10C depicts a flowchart describing an example of an embodiment ofprocesses for reading an array of phase change memory material basedmemory cells based on the write circuitry of FIGS. 5 and 10B. At step1040, the memory device receives a read request. In most memory systems,this will be from the controller in terms of physical addresses, but inother embodiments this can be a logical address that is converted to aphysical address by control circuitry on the memory device. The array isbiased for the sensing operation in step 1041, where the biasing canagain be as shown in FIG. 5 and described with respect to step 922 ofFIG. 9C, with a selected word line at a low voltage level, such asground, and the non-selected word lines at a high enough voltage toavoid current from a selected bit line flowing onto the non-selectedword lines. It should be noted that although the cross-sectional view ofthe vertical cross point structure illustrate in FIG. 7A bears someresemblance to a 3D NAND array (such as a BiCS structure), the cellsalong a bit line not connected in series and can individually beaccessed. This allows for any of the memory cells along a selected wordline to be read currently in a read process, so that a read page can bean entire word line or some portion of a word line, where the page sizecan be chosen based on performance and power considerations.

Once the array is based, the selected memory cells can be read in steps1042, 1043 and 1044. At step 1042, a sensing operation is performed byapplying a V_(read) value to the driver 1117 of FIG. 10B, where this canbe one of the read voltage levels V_(read1), V_(read2) or V_(read3) ofFIG. 10A for a 4-state per memory cell embodiment. After applyingI_(read) for some time, the result of the sensing operation can then bedetermined at step 1043 based on the V_(read_out) level and latched inthe latch circuits of data latch 568. Step 1044 determines if moresensing operation for other data levels or other memory cells are to beperformed and, if so, the flow loops back to step 1042 for the nextsensing operation with a shifted V_(read) value if a different state isto be sensed. When reading multi-state data, different orders forreading the data states can be used depending on the embodiment for theread algorithm. To take one example, the sensing operation on the firsttime through step 1042 could be for the lowest level of V_(read1),followed by V_(read2) the next time through and finally V_(read3),where, to save on power consumption, if a memory cells is found toconducting at one V_(read) value, it can be omitted from sensing insubsequent iterations. Once all of the levels to be sensed are sensed,the read result is output at step 1045.

FIGS. 11A-11I may depict various stages of fabrication usingcross-sectional views for the embodiment of FIGS. 7A-7C. FIGS. 11J-11Lshow a top view at a word line layer of a portion of one embodiment forvarious stages of fabrication for the embodiment of FIGS. 7A and 7B.FIGS. 11A-11L may be referred to when describing the process of FIG. 12.

FIG. 12 depicts a flowchart describing an example of an embodiment ofprocesses for forming portions of a memory array. The flowcharts mayomit common processing steps (e.g., the formation of isolation regionsor structures, various implant and annealing steps, the formation ofvias/contacts, the formation of a passivation layer, planarization,etc.) in order to highlight the processing steps described.

Referring to FIG. 12, in step 1202 global word lines GBL are formed overa substrate, as illustrated in FIG. 11A. The global bit lines may beformed over an n+ polysilicon layer or positioned above a substrate,such as a silicon substrate or glass substrate. In some embodiments, theregion under the lower dielectric layer and global bits lines canincludes CMOS or other circuit elements (decoders, drivers, and so on)for operation of the array. At step 1204, a layer of dielectric D isformed over the global bit lines GBL, as shown in FIG. 11B, and at 1206an opening etched to expose the global bit lines where the verticallocal bit lines will later be located, with the resultant structureshown in FIG. 11C. A switch for connecting the global bit line GBL tothe corresponding subsequently formed local bit lines is formed at step1208. FIG. 11D shows the resultant structure for an embodiment using avertical thin film transistor (VTFT) for this switch, but other switchtypes can be used.

In step 1210, an alternating stack of word line layers WL and dielectriclayers D are formed. The alternating stack of word line layers anddielectric layers may comprise an alternating stack of TiN orpolysilicon that are separated by layers of oxide or silicon dioxide.The alternating stack of word line layers and dielectric layers may beformed over the previously formed one or more global bit lines or abovea global bit line layer. The alternating stack of word line layers anddielectric layers may be deposited using various deposition techniquessuch as chemical vapor deposition (CVD), physical vapor deposition(PVD), or atomic layer deposition (ALD). FIG. 11E illustrates an exampleof this structure.

In step 1212, a memory hole or trench is etched extending through thealternating stack of word line layers and dielectric layers. The memoryhole or trench may be etched using various etching techniques such asdry etching, wet chemical etching, plasma etching, or reactive-ionetching (ME). An example of the resultant structure is shown in FIG.11F. Depending on the embodiment, the word line regions on either sideof a trench may be separate or connected in the case of a memory hole.The word line layers may comprise a layer of TiN, polysilicon, titanium,tantalum, or tungsten (W). The dielectric layers include dielectriclayer may comprise a layer of silicon dioxide.

In an embodiment using “crested” word line structure (word lines withextending prongs), after step 1212 the top view corresponding to that ofFIG. 7B is shown in FIG. 11J. Each of the word lines has a series ofprongs extending outwards towards the memory hole regions in which thevertical bit line structure will later be formed.

The prongs, such as 1101-1107, can be formed in a variety of ways. Inone embodiment, the structure shown in FIG. 11J is formed by initiallyforming the word lines shown running horizontally, the verticalconnectors for the comb structures with prongs such as 1101-1107 andsections that run vertically in FIG. 11J. The sections runningvertically in FIG. 11J can be divided to form the prongs 1101-1107 byetching memory holes in the locations indicated that are configured toremove word line material in the memory hole and form prongs 1101-1107.Alternately, in other embodiments the prongs 1101-1107 can be formedusing masking techniques when the word line combs 1101-1107 are formed.

Referring to FIG. 12, in step 1214, a portion of the word line layers isrecessed by etching the word line layers from the memory holes back fromthe memory hole relative to the dielectric layers on either side. Theword line layers may be recessed using a selective etch that targets thematerial of the word line or a timed etch which etches the material ofthe word line at a faster rate than the material of the dielectric.Referring to FIG. 11G, a portion of the word line layers has beenrecessed or removed, to form a pocket recessed relative to a verticalface of the dielectric layers.

In embodiments using the “crested” word line structure, after step 1214the top view corresponding to that of FIG. 7B is shown in FIG. 11K. Thistop view is at a word line layer, where the prongs have now been etchedback in the word line layer to form the recessed pocket within thesurrounding dielectric layers, both above and below (in the verticaldirections) and to the sides (laterally, separating adjacent memorycells on the same word line).

In step 1216, the portion of the word line layers that was recessed toform a pocket is lined with a surfactant, such as a metal-nitride.Referring to FIG. 11H, the portion of the word line layers has beenfilled with surfactant in each of the word line pockets, such as by thedeposition of a conformal surfactant and subsequent etch to remove anysurfactant on side-walls of memory hole, where this subsequent etch canbe omitted in some embodiments if the resistivity of the surfactant isnot too low. The surfactant layer can be applied by, for example, anelectroplating process, an atomic layer deposition (ALD), anElectrochemical Atomic Layer Deposition (E-ALD), or other processes.FIG. 11H again shows the underlying global bit line GBL and the verticalthin film transistor VTFT of one embodiment.

In step 1218, the recessed pocket region in the word line is filled bythe deposition of a conformal phase change memory material to form thememory cell. Depending on the embodiment, the phase change material canbe deposited by, for example, using an electroplating process, an atomiclayer deposition (ALD), an Electrochemical Atomic Layer Deposition(E-ALD), or other processes. In filling the pocket region to form theconfined PCM material of the memory cell, the PCM material may have alsofilled in the trench or memory hole region as well, where the amount towhich this occurs will be process dependent. The trench or memory holemay then need to be cleaned up to re-establish the trench or memory holeregion by removed the PCM material to form the individual memory cellswith in each of the recessed pockets before forming the vertical bitline structure. Once the memory hole or trench region is cleaned up, inembodiments that include the conformal carbon layer C, this is thendeposited in step 1220. The carbon layer C is substantially aninsulating layer, since although it should allow current to pass betweenthe PCM material and the ovonic threshold switch layer, if the C layeris not sufficiently resistant it can short the memory cells on differentlayers together without going through the PCM material.

Formation of an ovonic threshold switch layer OTS follows in step 1222,which is then etched followed by the deposition of the vertical bit lineVBL in step 1224. An example of the resultant structure is shown incross-section in FIG. 11I, which correlates to FIG. 7A. For embodimentsusing a “crested” word line structure, the top view of the resultantstructure is shown in FIG. 11K, which corresponds to FIG. 7B. Thevertical bit line VBL of step 1224 can be a conducting material (e.g.,tungsten) that is deposited within the trench in one embodiment, ormemory hole in another embodiment.

The various embodiments described above provide for a non-volatilememory using a phase change memory material with increased storagedensities. More specifically, the vertical cross-point structure allowsfor a three-dimensional array of memory cells and the use of asurfactant allows for more reliable multi-state storage.

In some embodiments, a method of forming a non-volatile memory structureincludes forming alternating horizontal layers of a plurality ofdielectric layers and one or more word lines layers and etching avertical region through the alternating horizontal layers of word linelayers and dielectric layers. The word lines are etched horizontallyfrom the vertical region to form regions recessed from the verticalregion within the dielectric layers. A phase change memory material iswithin the recessed regions and a vertical bit line is formed within thevertical region.

In further embodiments, a method includes forming a first dielectriclayer on a substrate and forming a patterned conductive layer on thefirst dielectric layer, the patterned conductive layer having aplurality of prongs extending horizontally relative to the substrate. Asecond dielectric layer is formed over patterned conductive layer thesecond dielectric layer is etched into vertically relative to thesubstrate to expose the ends of the prongs. The exposed ends of theprongs are etched block horizontally relatively to the substrate to froma corresponding plurality of pores in the first and second dielectriclayers. A phase change memory material is formed within the pores andone or more conducting lines running vertically with respect to thesubstrate are formed, the phase change memory material within each ofthe pores connected between the conductive layer and one of theconducting lines as part of a memory cell.

In additional embodiments, a method includes forming a word line runninghorizontally between dielectric layers and forming a bit line runningvertically through the dielectric layers. A memory cell is formedconnected horizontally between the word line and the bit. Forming thememory cell includes forming a pocket within the dielectric layersexposing the word line; forming a phase change memory material withinthe pocket configured to provide a first current path between the wordline and the bit line, the phase change memory material having a firstphase with a first resistivity and a second phase with a secondresistivity, the first resistivity being higher than the secondresistivity; and forming an intermediate resistance material adjacent tothe phase change memory material within the pocket configured to providea second current path is parallel with the first current path betweenthe word line and the bit line, the intermediate resistance materialhaving a resistivity higher than the second resistivity and lower thanthe first resistivity.

For purposes of this document, a first layer may be over or above asecond layer if zero, one, or more intervening layers are between thefirst layer and the second layer.

For purposes of this document, it should be noted that the dimensions ofthe various features depicted in the figures may not necessarily bedrawn to scale.

For purposes of this document, reference in the specification to “anembodiment,” “one embodiment,” “some embodiments,” or “anotherembodiment” may be used to describe different embodiments and do notnecessarily refer to the same embodiment.

For purposes of this document, a connection may be a direct connectionor an indirect connection (e.g., via another part). In some cases, whenan element is referred to as being connected or coupled to anotherelement, the element may be directly connected to the other element orindirectly connected to the other element via intervening elements. Whenan element is referred to as being directly connected to anotherelement, then there are no intervening elements between the element andthe other element.

For purposes of this document, the term “based on” may be read as “basedat least in part on.”

For purposes of this document, without additional context, use ofnumerical terms such as a “first” object, a “second” object, and a“third” object may not imply an ordering of objects, but may instead beused for identification purposes to identify different objects.

For purposes of this document, the term “set” of objects may refer to a“set” of zero or more of the objects.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

What is claimed is:
 1. A method of forming a non-volatile memorystructure, comprising: forming alternating horizontal layers of aplurality of dielectric layers and one or more word line layers, theword line layers each including one or more prongs extendinghorizontally between the dielectric layers; etching a vertical regionthrough the alternating horizontal layers of word line layers anddielectric layers exposing ends of the prongs; etching the prongs of theword line layers horizontally from the vertical region to form regionsrecessed from the vertical region within the dielectric layers, therecessed regions surrounded by the dielectric layers above, below andlaterally on both sides; lining the recessed regions with a surfactant;forming a phase change memory material within the recessed regions linedwith the surfactant, the phase change memory material formed within therecessed region lined with the surfactant such that the phase changematerial is separated from the surrounding dielectric layers and theword line layers by the surfactant; and subsequent to forming the phasechange memory material within the recessed regions, forming a verticalbit line within the vertical region.
 2. The method of claim 1, whereinthe surfactant comprises a metal-nitride.
 3. The method of claim 1,wherein lining the recessed regions with the surfactant includes anelectroplating process.
 4. The method of claim 1, wherein forming thephase change memory material within the recessed regions includes anelectroplating process.
 5. The method of claim 1, wherein the phasechange memory material is a chalcogenide material.
 6. The method ofclaim 5, wherein the chalcogenide material is Ge₂Sb₂Te₅ (GST).
 7. Themethod of claim 1, further comprising: forming a selector switch betweenthe phase change memory material in each of the recessed regions and thevertical bit line.
 8. The method of claim 7, wherein forming theselector switch between the phase change memory material in each of therecessed regions and the vertical bit line includes forming an ovonicthreshold switch material between the phase change memory material andthe vertical bit line.
 9. The method of claim 8, further comprising:forming a diffusion barrier layer between the phase change memorymaterial and the ovonic threshold switch material.
 10. The method ofclaim 9, where the diffusion barrier layer includes a carbon layer. 11.The method of claim 1, further comprising: forming a global bit line ona substrate; and forming a switch, wherein the alternating horizontallayers of word line layers and dielectric layers are formed over theglobal bit line and the switch and the switch connects the bit line tothe global bit line.
 12. The method of claim 11, wherein the switch is avertical thin film transistor.
 13. The method of claim 1, wherein: thephase change memory material has a first phase with a first resistivityand a second phase with a second resistivity, the first resistivitybeing higher than the second resistivity; and the surfactant is anintermediate resistance material, having a resistivity higher than thesecond resistivity and lower than the first resistivity.
 14. The methodof claim 13, wherein, for each of the recessed regions, the liningsurfactant is configured to provide a current path parallel to thatprovided by the phase change memory material formed therein between thevertical bit line and a corresponding one or the word line layers.
 15. Amethod, comprising: forming a word line running horizontally betweendielectric layers, the word line including one or more prongs extendinghorizontally between the dielectric layers; subsequent to forming theword line, forming a bit line running vertically through the dielectriclayers; and subsequent to forming the word line and prior to forming thebit line, forming a memory cell connected horizontally between the wordline and the bit line, wherein forming the memory cell comprises:forming a pocket within the dielectric layers exposing the word line byetching back the prongs of the word line, the pocket surrounded by thedielectric layers above, below and laterally on both sides; forming aphase change memory material within the pocket configured to provide afirst current path between the word line and the bit line, the phasechange memory material having a first phase with a first resistivity anda second phase with a second resistivity, the first resistivity beinghigher than the second resistivity; and prior to forming the phasechange memory material within the pocket, forming an intermediateresistance material within the pocket adjacent to the phase changememory material within the pocket and separating the phase changematerial from the surrounding dielectric layers, the intermediateresistance material configured to provide a second current path which isparallel with the first current path between the word line and the bitline, the intermediate resistance material having a resistivity higherthan the second resistivity and lower than the first resistivity. 16.The method of claim 15, further comprising: forming a selector switchbetween the parallel first and second current paths and the bit line.17. The method of claim 15, wherein forming the phase change memorymaterial within the pocket includes an electroplating process.
 18. Themethod of claim 15, wherein the intermediate resistance material isformed within the pocket such that the phase change memory material isseparated from the dielectric layers and the word line by theintermediate resistance material.